The one vital thing you are missing is a lightsource, otherwise this is kinda cute atm
lil fair ground aww.. Don't ask me about palettes, im still figuring that out myself.
Don't add anything too noticable, like a muddy patch. things naturally erode, get used. tyre skid marks on the track? add some foliage, flowers, few rocks or grassy bits on the beach, a poster or two in the enterance. I went to a waterpark on holiday last week, there were sitting areas, sunbeds and walls eitherside of the paths and yes were there's water and grass there will be muddy patches, but don't make them too noticable (im pituring you drawing a brown blob somewhere). just a discolouring of the grass around, with breaks of dirt.
er well, goodluck with this, your still working on this so these are just suggestions.